The photochemical and thermal stability of the detergent-solubilized blue- and green-absorbing proteorhodpsins, BPR and GPR, respectively, are investigated to determine the viability of these proteins for photonic device applications. Photochemical stability is studied by using pulsed laser excitation and differential UV-vis spectroscopy to assign the photocyclicity. GPR, with a cyclicity of 7 x 10(4) photocycles protein(-1), is 4-5 times more stable than BPR (9 x 103 photocycles protein(-1)), but is less stable than native bacteriorhodopsin (9 x 10(5) photocycles protein(-1)) or the 4-keto-bacteriorhodopsin analogue (1 x 10(5) photocycles protein(-1)). The thermal stabilities are assigned by using differential scanning calorimetry and thermal bleaching experiments. Both proteorhodopsins display excellent thermal stability, with melting temperatures above 85 degrees C, and remain photochemically stable up to 75 degrees C. The biological relevance of our results is also discussed. The lower cyclicity of BPR is found to be adequate for the long-term biological function of the host organism at ocean depths of 50 m or more.